usual rack is provided for moving the saddle seme distance along
Turning now to the screwing gear, j is a rest for the screwing
head, with screw for adjustment, and when in position for work is
held by the handle N. At the same time a lever L, provided with
a screwed die fitting in the threads of the screw M, is placed at the
other end of the shaft H, so that when the screwing tool is on the
work, L engages with the screw M, but if the rest j be lifted and
thrown back, L is at the same time released. When in operation
the screw M is rotated from the mandrel by gearing of 2 : i, so that
a screw is cut at c, having half the pitch of the copy and of
reverse hand, M being usually left-handed, and c right-handed.
Of course the shaft H is capable of longitudinal motion, and the
piece M, being hollow, can be removed, and another of different
pitch applied, while the die, usually made of copper or soft brass,
does not need special cutting, but will find its way into the threads
of the screw.
Lastly, the lathe is provided with a hollow mandrel, which is
very useful for small articles that can be cut from a continuous
bar. An example of such work is shewn in progress, being the
making of a small tap bolt A hexagonal bar is held in a con-
centric chuck, drawn forward to a convenient length, and the
roughing tool g first applied, traversing to the front for position.
The bolt being roughed down, is finished by the tool h, and has its
end rounded by /. Next the screwing is performed by bringing
over the tool m ; and, lastly, the chamfering and parting are done
respectively by the tools k and /. It will be, therefore, clear that
a great deal of time and labour may be saved by the use of such
a tool where articles have to be made in quantity. All bolts and ;
studs are turned at such a lathe. (See Appendices, pp. 814, 978, I
* Erector's Tools,—These must include Lifting Tackle and a |
Portable Drilling tool. The latter is known as the Ratchet!
Brace, and is shewn at Fig. 215 in position for drilling a hole, j
The pillar A is clamped to the work, and carries an arm F, which |
can be set at various heights, to take the brace and drill a As j
the latter is ground to cut in one direction only (see d, page 168), !
the brace is made to enclose a ratchet wheel c fixed to the drill i